Recording inks, comprising a dye, a large proportion of water, and water-miscible solvents, have been used in thermal ink-jet printers. Such inks, however, must meet rather stringent requirements. For example, they should be stable over a prolonged period, have the correct viscosity, conductivity, and surface tension, and also rapidly fix onto the recording surface. Further, the resultant image should be clear and have good fastness to light and water.
The vehicles used in the foregoing formulations are generally mixtures containing significant amounts of water. Such formulations, however, cannot be used effectively to mark plastics; they smear, blur, and/or wash off easily. Presently, plastics used in keyboard keycaps are either marked by printing a template, pressing the template against a plastic substrate, and heating the combination to a temperature from about 150.degree. to about 250.degree. C., or by sublimation method. All these requires plastics that retain their dimensionality at the elevated temperature. In contrast, the present invention involves direct printing and avoids heating and the need for a template.